Hot melt adhesive applicators currently available have their heating elements powered from a commercial alternating current source with control of the temperature of the heat block to melt the adhesive being obtained by the use of a bimetal-type thermostatic switch. The thermostatic switch is mounted within the applicator body to respond to the temperature of the heat block. While temperature adjustable bimetal-type thermostats are available for higher power applications (above about three amperes), they are large and tend to be expensive. Also, adjustment accuracy is poor. For these reasons, hot melt adhesive applicators that use bimetal-type thermostatic temperature control are almost entirely single temperature devices. The typical approach for a multi-temperature applicator is to have two or more single temperature thermostats that are switch selectable. Even then a bimetal-type thermostatic switch does not provide very close control of the nozzle temperature. A selected temperature setting can be expected to typically vary .+-.8.5.degree. C. to .+-.14.degree. C. There is need for a temperature control system that has a much reduced swing about a selected temperature setting, since a smaller temperature swing will allow use of a higher wattage heating element to reduce the time needed to attain a desired temperature allowing more adhesive to be applied by an applicator for a given time period. A smaller temperature swing will also produce more consistent and desirable flow rates for an adhesive to provide a desired application result. It is also desirable that the selection of various temperature settings be carried out in a manner that is repeatable.
A triac heater control integrated circuit (IC) is shown in application literature published by the Motorola, Inc., Phoenix, Ariz. relating to Motorola's type CA 3079 zero voltage switches. This circuitry is suitable for control of temperature at one setting, but can present an uncontrolled mode of operation when it is desired to control the temperature maintained by operation of a heater element at more than one level. If the control circuitry is positioned for control at one temperature level and the control level is changed to a temperature at a lower level that is sufficient to cause the voltage at the positive input of a differential amplifier to go sufficiently negative relative to the voltage at the negative input, the differential amplifier operates to provide a signal that causes the triac or thyristor of the circuitry, which controls the flow of current through a load (such as a heating element) to be turned on and remain on and thus cause a run away condition.
A Motorola type CA 3059 zero voltage switch is available and is like the CA 3079 but with a protection circuit provided. The protection circuit serves to remove the drive current from the triac or thyristor used with the CA 3059 device if the sensor used with the CA 3059 opens or shorts. The application literature places a restriction on employment of the protection circuit. It requires the temperature sensing element and an associated resistor, which with the sensing element provides a voltage divider, to have resistance values between 2,000 and 100,000 ohms. This restriction places a limit on the range of temperature settings that can be used. For example, if temperature control is desired between about 265.degree. C. and 99.degree. C. and a .phi.,000 ohm (at 25.degree. C.) thermistor is selected as the sensor to take advantage of the high end of the resistive limit, the thermistor will present a resistance around 75 ohms when the control circuit is set to control at 265.degree. C. causing the current drawn by the thermistor circuit to be too large for the power supply provided as a part of the device to maintain a workable output voltage level.